I remember your thread. I think I posted the contact details of support group Straight Partners Anonymous on there.
It's not uncommon for people to be in denial about their sexuality if they've been trying to ignore it for a long time.
I was in a nineteen year relationship with a man who maintained right up until we separated that he was not gay.
He's now in a LTR with another man. I've heard DC have say things like "daddy's gay but he didn't know" so I assume he now identifies as gay, but he's still never told me that.
I think my ex did actually know. He told me quite a lot shortly before we separated and he had been attracted to other boys from puberty.
I actually exchanged messages through MN with a lesbian who was also on a thread about either a gay husband or a lesbian wife (in a straight marriage). It was quite helpful to me because she clearly genuinely didn't know why her relationships with men hadn't worked out until she fell for another woman.
So, it's possible that your husband is gay and believes it when he says he isn't.
My marriage wasn't sexless, but it was lacking in affection and sex was very regulated. I think he needed sex to fit into a schedule so that he could be mentally prepared for it. I'm in a relationship with a lovely straight man now, and it's a very, very different experience.
I'm sorry you haven't got to the bottom of the anonymous message. Ultimately, regardless of whether or not your husband is gay, if you are not happy in the relationship then you don't have to stay in it.
From all the personal stories of straight women in relationships with gay men, it's pretty common for the man to take a long time to come out (if at all) after confronting his sexuality. Lots of men deny it for years, even if there is actually a lot of evidence (porn, affairs). So asking him isn't by any means going to give you a reliable answer. I asked my ex several times over the years and he always denied it.
None of that means that your husband is gay. But a lot of what you described fits with it being the case.
Don't stay in a relationship that isn't working for you. It's not worth it. Although I'm in a really good relationship now, I would far rather be single than in a loveless relationship like my marriage to my gay ex.